Gun is a Remmy 700, Im 99% sure it started life as a PSS.
I got in the Sinclair chamber length gauge and checked my chamber 5 times.
All were 2.042, except 1 being 2.043.
So should I trim to 2.040? Or is there a better formula for figuring this out?

Im not looking for ultra ultra BR 1000yd bullets. Yet.
Just trying to get as accurate as possible while I am still learning.
Thanks.

The max length for 308 brass is 2.015". If you are past that and your rifle likes it then you are kind of on your own. I am trimming mine to 2.005" and working on loads.

I would probably trim to 2.030 just to add room for error. I made a gage for my 700 and I too was getting about 2.045". My brass is short yet but I plan to let the length grow.

I see what you are saying, and it makes sense.
The case length doest matter much like the bullet seating.
.001 in the lands, .025 out of the lands,,,,,,,,,,,,,,etc, etc.

Sorry for the bonehead question.
Need more coffee.

If you check the SAAMI chamber drawing  here ,  2.0488" is maximum, but with a neck diameter of .310"   This is the "crimp" area that will lock you bullet into the  case, raising pressure.   KABOOM   Having measured my 243win with a home made tool, i think this is a non-issue for accuracy and can be very dangerous if you dont know what your doing. I have measured shoulder set back on a 223 at .006" from the firing pin strike. If the 308 shoulder is set back of firing, this will let more neck length protrude into the chamber. Not what i want to happen if trim length is over the maximum.  

   Link to Quote  If we subtract .024" from  2.0488"  = 2.0248"   This comes close to the  2.025" chamber drawing length, with a neck diameter of .344"    So, my guess would be-Maximum trim length is  2.025" With a trim to length of 2.015"      If your feeling lucky? Use this information at our own risk.  

I have that tool,  it comes with pretty clear directions on how much below your number to trim to.  


Sinclair says your length measured in your gun minus 0.024" and to maintain that minimum over the life of the brass.


Can you get away with getting closer to the end of the neck?  I wouldn't doubt it but I would think it through very carefully.  Things like am i shooting a fireformed neck sized bench rest setup where I don't expect a lot of movement  or are we talking a FL sized everytime with the  shoulders pushed back 0.005".  OAL is one thing but for this examination I would say the distance from the datum line to the end of the neck is as or more important.    Just food for thought.

Well now I am even more confused.

It's not that hard, just think it through with me.  

Bottle neck cases headspace off the shoulder right?  
Resizing, controls the diameter of the casing and the shoulder location.  You adjust the shoulder location by how much or how little you screw in the die.
In theory you could have the total case length within limits but push the shoulder too far back in effect making the neck longer above the shoulder.  If you put this theoretical casing into your gun the spring loaded ejector will hold it forward, case shoulder on the chamber shoulder.  Your theoretical long neck is getting closer to the end of the chamber (and you would have more of a gap (headspace between the primer and the bolt face).

The main thing to avoid is having your casing mouth enter the chamber throat as it's not designed to have brass there; not enough room for the brass to expand and let go of the bullet.  Any pinching or holding at that point will spike pressure.  That's why sinclair is telling you to stay well back from the rifle's end of chamber.  How close can you get?  I can't exactly say for sure.  I'm sure you could get more input from bench rest shooters as I suspect they're probably a lot closer than i am.

One alternative is to trim to the loading book max.  You're still within spec and probably still have a good margin of safety.  

Just my $0.02 more.

Your  measurment of 2.042" minus Sinclair's standoff of 0.024" is  2.018".

That's 3 thousandths over the book max case length.    I'd go with that and call it good.

Signal25, this should work safely for you.  

Consistency means more than anything (within extremes of course). The actual difference between "book" trim length and 2.040" is .035" There is no way .035" of neck length is going to make a significant difference in anything load related.

I used to shoot a .280 Rem caliber match rifle. My brass was GI 06' converted to .280 The 06' case is about .040" shorter than the .280's

I trimmed them all to 2.500" neck only sized and seated the bullets to within .010" of the lands. The rifle shot great.

I just don't see any real reason or benefit to exceed the recommended max trim length.